1. Field of the Invention
The invention is directed to the field of flexible materials and, in particular, to the field of flexible composite materials. The material has direct application to inflatable structures such as the gas bag for lighter-than-air vehicles.
2. Description of Related Art
In large non-rigid lighter-than-air vehicles, the material used for the gas bag must meet a large number design requirements such as high strength, provide tear resistance, act as a gas barrier, not be subject to degradation by the environment including ultra violet radiation due to exposure to sunlight. Thus such a material winds up being a multi-layer laminate combining materials with diverse properties. The primary axial loads on any portion of the wall of the gas bag are at 0 degrees to the longitudinal axis of the gas bag and at 90 degrees thereto (circumferential). Thus most laminates include woven filamentary material with the filamentary material orientated at the 0 and 90 degree angles. Additionally, to carry shear loads, filamentary material is sometimes included with orientations at plus or minus 45 degrees to those carrying the axial loads.
In early designs, where stress levels were low, several layers of woven cotton cloth impregnated with rubber to provide the gas seal were often used. Later, artificial fibers such as RAYON.TM. or DACRON.TM. were used, manufactured by the E. I. duPont de Nemours & Company (hereinafter referred to as "DuPont"). The layers of cotton cloth were at 0 and 90 degrees (axial or strength plies) for the tension loads and plus and minus 45 degrees (bias plies) for the shear loads. However, this approach did not always result in an optimal strength design for the strength required to carry the shear loading was typically, much less than the capability of the bias plies. Using the same material for both the axial tension loads as well as the bias (shear) loads often resulted in a weight penalty.
Some modem designs use a woven polyester fiber such as DACRON.TM. for the 0 and 90 degrees axial load carrying material. A film of material that is impervious to Helium such as a polyester terephthalate that serves as the gas barrier is also carries some shear load. A typical polyester terephthalate is sold by DuPont under the trade name MYLAR.TM.. Woven polyester fiber such as DACRON.TM. has a very large strain to failure value, about 20 percent. However, in large non-rigid airships, the strength requirements have dictated the use of very high strength materials such as a liquid crystal thermotropic (melt spun) polyester polyarylate fiber, for example VECTRAN.TM. manufactured by Hoechat Celanese, Germany to carry the axial loads. Another high strength material is a lyotropic (solvent spun) aromatic polyaramide fiber, such as KEVLAR.TM., which is manufactured by DuPont. However, both VECTRAN.TM. and KEVLAR.TM. have a very small value of strain to failure value, on the order of 4 percent. If the bias layers where made of the same material, biaxial loading in the 0 and 90 degree fibers will transfer significant load to the 45 degree bias layers. Requiring these layers to work as hard as the 0 and 90.degree. plies, introduces a potential failure mode, or a weakening of the system. In fact having a bias layer with higher elongation than the 0 and 90 degree (strength fibers) precludes premature failure in the bias ply at ultimate load in the strength fibers.
Some of the prior art teaches away from the use of such a concept, for example, German Patent No. DE 3702936 "Fiber Composite Material-With high Tensile And High Modulus Fiber In different Orientations by S. Roth, et al. Roth, et al. teaches the use of fibers with high strength and elongation at 0 and 90 degrees in conjunction with 45 degree fibers that have a high elastic modulus for use in rigid composite structures. Thus the stain value at failure of the plus or minus 45 degree fibers is less than the 0 and 90 fibers.
In U.S. Pat. No. 4,770,918 "Diagram For Producing Sound" by A. Hayashi a flexible diagram for producing sound is disclosed having at least one layer of a first woven fabric having a low elongation and at least two layers of a second woven fabric having a high elongation. The first and second fabrics are disposed in such a fashion that the warps thereof cross each other at between 10 and 80 degrees whereby an elongation of the diaphragm in the direction of the warps of the first fabric is generally equal to the elongation of the diaphragm in a direction inclined at a 45 degree angle relative to the direction of the warps of the first fabric. This allows for ease of tuning of the diaphragm. This invention, of course, would produce an inefficient pressurized structure.
Other patents of general interest wherein materials of different properties are combined into a single flexible structure are U. S. Pat. Nos. 5,189,280 "Three Dimensional Fiber Structures Having Improved Penetration Resistance" by G. A. Harpell, et al., 4,871,598 "Container With Flexible Walls" by E. Potente, et al. and 5,215,795 "Shock-Absorbing Air Bag" by M. Matsumoto, et al.
Thus it is primary object of the invention to provide a laminate material suitable for the wall of flexible pressurized containers.
It is a another primary object of the invention to provide a laminate material suitable for the wall of a flexible pressurized containers wherein bias shear load carrying plies have a greater strain to failure value than the axial tension load carrying plies.
It is a further object of the invention to provide a laminate material suitable for the flexible wall pressurized containers that is not degraded by ultra violet radiation.
It is a still further object of the invention to provide a laminate material suitable for the wall of flexible pressurized containers that are suitable for containing Helium gas.
It is another object of the invention to provide a laminate material suitable for the wall of flexible pressurized containers that can easily be seamed together.